(1) Field of the Invention
This invention relates to a capacity control valve for a variable displacement compressor, and more particularly to a capacity control valve for use in a variable displacement compressor for compressing a refrigerant gas in a refrigeration cycle of an automotive air conditioner.
(2) Description of the Related Art
A compressor used for compressing refrigerant in a refrigeration cycle of an automotive air conditioner is driven by an engine, and hence is not capable of controlling the rotational speed thereof. For this reason, a variable displacement compressor capable of changing the compression capacity for compressing refrigerant is employed so as to obtain adequate refrigerating capacity without being constrained by the rotational speed of the engine.
In such a variable displacement compressor, compression pistons are connected to a wobble plate fitted on a shaft driven for rotation by the engine, and the angle of the wobble plate is changed to change the stroke of the pistons for changing the discharge amount of the compressor.
The angle of the wobble plate is continuously changed by introducing part of the compressed refrigerant into a gastight pressure-regulating chamber and changing the pressure of the introduced refrigerant, thereby changing a balance between pressures applied to the both ends of each piston.
To control the amount of refrigerant introduced into the pressure-regulating chamber of the variable displacement compressor, in a compression capacity control device described e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2001-132650, there have been proposed a construction in which a capacity control valve is disposed between a discharge chamber and a pressure-regulating chamber of the variable displacement compressor, and an orifice is provided between the pressure-regulating chamber and a suction chamber, and a construction in which an orifice is provided between a discharge chamber and a pressure-regulating chamber, and a capacity control valve is disposed between the pressure-regulating chamber and a suction chamber.
Each of the capacity control valves opens and closes the communication between the chambers such that a differential pressure across the capacity control valve is maintained at a predetermined value, and the capacity control valve is implemented by a solenoid control valve capable of externally setting the predetermined value of the differential pressure by a current value. Thus, when the engine rotational speed increases, the capacity control valve is opened between the discharge chamber and the pressure-regulating chamber, or the capacity control valve is closed between the pressure-regulating chamber and the suction chamber, whereby the pressure introduced into the pressure-regulating chamber is increased to reduce the volume of refrigerant that can be compressed, while when the engine rotational speed decreases, the capacity control valve is reversely controlled such that the pressure introduced into the pressure-regulating chamber is decreased to increase the volume of refrigerant that can be compressed, whereby the pressure of refrigerant discharged from the variable displacement compressor is maintained at a constant level irrespective of the engine rotational speed.
However, in the conventional capacity control valve for the variable displacement compressor, not only the capacity control valve but also an orifice is arranged in the passage leading from the discharge chamber to the suction chamber via the pressure-regulating chamber of the variable displacement compressor, and the orifice is determined by taking into account the amount of leakage of refrigerant from the discharge chamber to the suction chamber. Actually, however, it is difficult to set an appropriate size of the orifice due to varied manufacturing tolerances of the variable displacement compressor. Further, the variable displacement compressor is controlled such that the differential pressure between a discharge pressure and a suction pressure is held constant. However, since the capacity control valve in charge of the control is inserted between the pressure-regulating chamber and the discharge chamber or the suction chamber, the capacity control valve is sometimes adversely affected by the pressure from the pressure-regulating chamber during capacity control operation.